In traditional cylinder lubrication systems, mainly for large 2-stroke diesel engines, use is made of one or more central lubricators, each of which serves the lubrication points on a single or several cylinders, i.e. by the pressure-feeding of portions of oil through respective connection pipes to the various lubrication points at relevant intervals of time. See e.g. DK/EP 0678152. These relevant intervals can typically be when the piston rings are disposed opposite the relevant lubrication point during the compression stroke when the piston is moving upwards.
It has proved, however, that the compressibility of the amount of oil arising in the pipes has made it difficult to establish this correct xe2x80x9ctimingxe2x80x9d. The length of the oil pipes used in practice is often so great that the introduction of a relatively small amount of oil in the one end of the pipe merely gives rise to a compression of the oil in the pipe without the pressure being great enough to press a corresponding amount of oil out of the other end at the cylinder surface. The oil is often not dosed at the above-mentioned time, but instead at times when the pressure in the cylinder is sufficiently low, as a rule after the passage of the piston in the upwards or downwards direction. If this occurs during the downwards movement, the oil is distributed over the surface of the cylinder from the lubrication point and downwards in the cylinder lining instead of upwards towards the xe2x80x9chotxe2x80x9d end of the cylinder where the lubrication is most necessary.
The development towards still greater utilisation of the engines has resulted in an increased mechanical and thermal load on cylinder linings and piston rings, which traditionally is accommodated with an increase in the dosing of cylinder oil. It has proved, however, that if the dosing is increased in excess of a certain limit, which is not defined, the speed at which the oil is introduced into the cylinder is so great that instead of remaining on the surface of the cylinder, it forms a jet inside the cylinder cavity and is hereby lost. If the dosing is effected as desired, while the piston rings are disposed opposite the piston, this is not so critical, but if the dosing takes place outside this period as described above, there is no benefit gained from a part of the oil which is dosed.
The traditional manner in which oil is distributed over the surface of the cylinder has been to establish two inclined slots per lubrication point in the surface of the cylinder, both extending out from the lubrication point and in a direction away from the top of the cylinder. When a piston rings passes such a slot, a fall in pressure occurs in the slot across the piston ring, which presses the oil away from the lubrication point. However, this and other methods have proved to be inadequate, in that in practice a considerable variation can be ascertained in the wear along the circumference of the cylinder.
Therefore, it is relevant to seek methods of improving the distribution of oil over the cylinder periphery.
With the present invention, the oil is also dosed in portions at certain periods of time, but it is distributed over the surface of the cylinder before the piston passes the lubrication points during its movement upwards.
The scavenging-air ports in uniflow-scavenged 2-stroke diesel engines are disposed in such a manner that during the scavenging, the gas mixture is set in rotating movement at the same time that the gas is displaced upwards in the cylinder, and leaves this through the exhaust valve in the top of the cylinder. The gas in the cylinder thus follows a helical line or swirl on its way-from the scavenging-air ports to the exhaust valve. Due to the centrifugal force, a sufficiently small particle of oil which exists in this swirl will be forced out towards the cylinder wall, and will finally be deposited on the wall. This effect is utilised by introducing the portions of oil into the cylinder as a xe2x80x9cmistxe2x80x9d of oil particles of suitable size, atomised through nozzles. By adjusting the dimensions of the nozzles, the outflow speed of the oil and the pressure before the nozzle, it is possible to control the average size of the drops of oil in the oil mist. If an oil particle or drop of oil is too small, it will xe2x80x9cfloatxe2x80x9d too long in the gas flow, and eventually be led away with the scavenging-air without impinging on the wall of the cylinder. If it is too large, due to its inertia it will continue too long in its initial path and not reach the cylinder wall, the reason being that it is overtaken by the piston and is deposited on the top of the piston.
The direction of the nozzles in relation to the flow in the cylinder can be arranged so that interaction between the individual drops of oil and the gas flow in the cylinder ensures that the drops of oil impinge on the cylinder wall over an area which corresponds by and large to the peripheral distance between two lubrication points. In this way, the oil is already distributed more or less uniformly over the cylinder surface before passage of the piston rings. Moreover, the nozzle will be able to be adjusted so that the oil impinges on the cylinder wall higher up than the nozzles. Consequently, already upon its introduction into the cylinder, the oil will not only be better distributed over the cylinder surface, but will also be xe2x80x9cdeliveredxe2x80x9d to the cylinder surface closer to the cylinder top, where the need for lubrication is greatest. Both of these conditions will result in a better utilisation of the oil, with an anticipated improvement in the cylinder lifetime/oil consumption relationship.
The feeding of the oil to the cylinder surface must be effected in measured portions, as is quite the case with the earlier-mentioned, traditional timed systems. The feeding means can be a traditional lubricator, but other feeding means with corresponding characteristics can also be envisaged.
In order to ensure that the pressure in the cylinder is not transmitted rearwards in the oil tube, a non-return valve is arranged in the normal manner in the end of the lubrication pipe, immediately in front of the inside surface of the cylinder lining. The non-return valve allows oil to pass from the oil tube to the cylinder lining, but does not allow the flow of gas in the opposite direction. These non-return valves normally have a modest opening pressure (a few bar).
The pressure which exists in the new system is necessary in the lubrication pipes between pumps and nozzles in order to ensure that the intended atomisation is considerably higher (in the order of 50-100 bar). If this were to be ensured by means of a considerable increase in the opening pressure of the traditional non-return valves, this will require stronger and more space-demanding springs, which will also result in greater xe2x80x9cinjurious spacexe2x80x9d between valve and nozzle. With traditional systems, this injurious space is already of the same magnitude as, or greater than, that amount of oil which must be dosed per portion, and therefore gives rise to a corresponding uncertainty with regard to the pressure in front of the nozzle. In order to ensure the necessary atomisation, it is necessary that the pressure required for the atomisation is available immediately upon the start of the dosing. This can be ensured, for example, by providing a valve where each of the oil tubes open out into the cylinder, and which is opened by the pressure in the oil pipe between the lubricator and the valve when this pressure has reached a certain value, such as is the case with traditional fuel oil injection systems.
Since the oil is supplied to the cylinder wall before the passage of the piston, the timing is not quite so critical as with systems where the oil must be fed precisely during the very short interval when the xe2x80x9cpackxe2x80x9d of piston rings is lying opposite the lubrication point.